This application is based upon Japanese Patent Application Nos. Hei. 11-272585 filed on Sep. 27, 1999, and Hei. 11-279971 filed on Sep. 30, 1999, the contents of which are incorporated herein by reference.
1. Field of the Invention
This invention relates to physical quantity detection apparatuses, and particular to a device for detecting the physical quantity such as acceleration, angular velocity, pressure or the like.
2. Related Art
Conventional capacitive physical quantity detection devices for detection of the physical quantity of the type which offer self-diagnosis capabilities include a capacitive acceleration sensor which is disclosed, for example, in Published Unexamined Japanese Patent application No. 8-110355. The capacitive acceleration sensor as taught thereby is designed to include a spring portion (beam portion) that is elastically deformable upon application of acceleration as the physical quantity to be detected, a movable electrode that is integrally formed with this spring, and a couple of stationary or fixed electrodes that are disposed to oppose this movable electrode, wherein a difference in capacitance between two capacitive elements as formed between the movable electrode and the fixed electrodes is subject to C-V conversion for measurement of an output.
The conventional capacitive acceleration sensor further includes an extra separate electrode in addition to the movable and fixed electrodes. This additional electrode is for use in performing the intended self-diagnosis and sometimes called xe2x80x9cself-electrodexe2x80x9d among those skilled in the art. Application of a certain voltage to this self-electrode permits creation of an electrostatic force, which in turn causes the spring portion to deform or offset in position for execution of the self-diagnosis required.
Unfortunately the conventional capacitive physical quantity capacitive detection device is encountered with a problem as to unwanted increases in overall size of the resultant device structure due to additional provision of the separate self-diagnosis electrode for establishment of the state of virtual creation of the physical quantity of interest.
This invention has been conceived in view of the background thus far described and its object is to provide a new and improved capacitive physical quantity detection device capable of avoiding the problem faced with the prior art.
It is another object of the invention to provide a capacitive physical quantity detector device capable of efficiently performing the intended self-diagnosis operation without having to additionally provide any separate electrode for exclusive use as a diagnosis electrode.
To attain the foregoing objects, in accordance with a first aspect of the present invention, a capacitive detection device is provided which includes a spring portion and a movable electrode and fixed electrode as integrally formed with the spring portion and operable to detect the physical quantity of interest in such a manner as to apply a periodically changeable signal between the movable electrode and the fixed electrodes and then derive an output voltage that is potentially variable in away pursuant to a change in capacitance value of a capacitive element as formed of the movable electrode and fixed electrode, characterized in that a detection signal for use in detecting the physical quantity and a self-diagnosis signal for use in performing self-diagnosis are selectively applied thereby permitting creation of a quasi-physical quantity at the movable electrode through deformation of the spring upon application of the self-diagnosis signal, and that the ratio of a frequency of the self-diagnosis signal to a resonance frequency of the spring in its deformation direction is specifically designed so that the resonance magnitude of this spring becomes more than one time upon application of the self-diagnosis signal.
First, in accordance with the instant invention, periodical application of the self-diagnosis signal between the movable electrode and the fixed electrode during self-diagnosis permits production of an electrostatic force between the movable electrode and fixed electrode, which in turn makes it possible to allow both the spring and the movable electrode integral therewith to deform causing the movable electrode to be set in the state in which the physical quantity is virtually generated. In this case, it is possible to perform the intended self-diagnosis through detection of a positional offset or deformation of the movable electrode based on an output voltage of a C-V converter circuit.
Note here that in cases where the frequency of the self-diagnosis signal is significantly different from the resonance frequency of the spring in the deformation direction thereof, movement or motion of the spring hardly keeps track of the frequency of the self-diagnosis signal. In other words the spring does not vibrate, which could result in establishment of the state of direct current (DC) voltage application. In contrast thereto, if the frequency of the self-diagnosis signal is close in value to the resonance frequency then the spring exhibits resonance, thereby making it possible to allow the spring to deform with a greater amplitude than in the case of DC-like deformation. In this way, applying the self-diagnosis signal for permitting vibration of the spring makes it possible to increase the quasi-physical quantity occurring at the movable electrode, which in turn enables achievement of an efficient self-diagnostic procedure required.
Also note that in this invention, the ratio of the frequency of self-diagnosis signal to the resonance frequency of the spring portion in its deformation direction is specifically designed so that the spring""s resonance magnitude (i.e. the one with a positional offset of the spring and movable electrode being normalized or standardized as xe2x80x9c1xe2x80x9d upon applying of a DC voltage) is more than or equal to one timexe2x80x94preferably, 1.1 times. With such an arrangement, it is possible to permit the spring to vibrate upon application of the self-diagnosis signal, which in turn makes it possible to achieve the intended efficient self-diagnosis procedure. Hence, in accordance with the invention, it becomes possible to provide the capacitive physical quantity detection device capable of efficiently performing self-diagnosis without requiring additional provision of any separate electrode for exclusive use with the self-diagnostic procedure.
In accordance with a second aspect of this invention, in the relation of the frequency of the self-diagnosis signal versus the resonance frequency of the spring portion in its deformation direction, the self-diagnosis signal frequency is specifically designed so that it is less than or equal to a limited value that is 1.41 times (preferably, 0.2 to 1.4 times) greater than the resonance frequency of the spring in the deformation direction thereof. With the setting of such relation, it is possible to force the spring to vibrate at its resonance magnitude of more than or equal to one time upon application of the self-diagnosis signal, thereby enabling increase in efficiency of the self-diagnosis. Due to this, it becomes possible to provide the intended capacitive physical quantity detection device capable of efficiently performing self-diagnosis without having to provide any separate electrode for exclusive use during the self-diagnostic procedure.
In accordance with a third aspect of the invention, in the relation of the frequency of the self-diagnosis signal versus the resonance frequency of the spring in its deformation direction, the ratio of the self-diagnosis signal frequency to the spring""s resonance frequency in its deformation direction is specifically determined to let the spring vibrate upon application of the self-diagnosis signal thereto. This makes it possible to permit the spring to vibrate with the resonance magnitude of more than or equal to one time when the self-diagnosis signal is applied thereto, which leads to an ability to provide the intended capacitive physical quantity detection device capable of efficiently performing self-diagnosis without requiring additional provision of any separate electrode dedicated to the self-diagnosis required.
In accordance with a fourth aspect of the invention, a capacitive physical quantity detection device is provided which is designed to detect the physical quantity of interest by applying a periodically changeable signal between a movable electrode and fixed electrode which are integrally formed with a beam portion and each of which has a beam-like shape extending in the same direction as the beam portion and then deriving an output voltage potentially variable with a change in capacitance value of a capacitor consisting of the movable electrode and fixed electrode while selectively applying a detection signal for use in detecting the physical quantity and a self-diagnosis signal for effectuation of self-diagnosis to thereby permit creation of a quasi-physical quantity at the movable electrode through deformation of the beam portion upon application of the self-diagnosis signal thereto.
A further feature of the invention is that a beam width at the movable electrode in its deformation direction is substantially the same as a beam width at the beam portion in the deformation direction thereof while at the same time causing rigidity or stiffness at the movable electrode in its deformation direction to be greater than stiffness at the beam portion in the deformation direction thereof.
In accordance with the invention, since during the self-diagnosis the self-diagnosis signal is periodically applied between the movable electrode and the fixed electrode for creation of an electrostatic force between the movable electrode and fixed electrode, it becomes possible to cause the beam portion and the movable electrode integral therewith to deform or offset in position in a specified direction at right angles to the elongate direction of the beam, which in turn makes it possible to establish the state in which the physical quantity is virtually generated at the movable electrode. In this case, it is possible to perform the self-diagnosis through detection of resultant deformation of the movable electrode on the basis of an output voltage of the C-V converter circuit.
During this self-diagnosis, an increase in deformation of the movable electrode would result in a likewise increase in change of capacitance value of the capacitor consisting of the movable and fixed electrodes, which in turn makes it possible to increase the output voltage thus enabling efficient self-diagnosis. One preferable approach to attaining this is to lighten the movable electrode through decrease in beam width at the movable electrode in its deformation direction. However, an excessive decrease in beam width can result in not only the beam portion but also the movable electrode per se being bent and offset in position upon application of the physical quantity.
Although in the capacitive physical quantity detection device of this type it is required that the deformation of the beam portion and that of the movable electrode be integral with each other and also in the same direction upon application of the physical quantity, deflection and offset in position of the movable electrode per se can result in an irregular change in distance between the movable and fixed electrodes, which often makes it impossible to obtain any desired capacitance change.
In contrast, the invention is such that the stiffness at the movable electrode in its deformation direction is made greater than the stiffness at the beam portion in its deformation direction while simultaneously making thinner to ensure that the beam width at the movable electrode in its deformation direction becomes substantially the same as the beam width at the beam portion in its deformation direction, which in turn makes it possible to prevent any unwanted deformation of the movable electrode upon application of the physical quantity while at the same time lightening the movable electrode.
Hence, in accordance with the present invention, it is possible to provide the capacitive physical quantity detector device capable of efficiently performing self-diagnosis without having to additionally provide any separate electrode for exclusive use in execution of the self-diagnosis. Note here that the beam width at the movable electrode in its deformation direction (movable electrode width), which is substantially the same as the beam width at the beam portion in its deformation direction (beam portion width) as has been stated supra, may be designed when reduction to practice so that the movable electrode width falls within a range of from 0.8 to 1.2 times of the beam portion width in view of the presence of possible fabrication errors during manufacture of the device.
Additionally, in case the fixed electrode is formed into a beam-like shape extending in almost parallel to the movable electrode while letting the movable electrode oppose the fixed electrode at a side face of the beam portion, if the beam width of such fixed electrode in the deformation direction of the beam portion is made substantially the same as the beam width at the movable electrode in its deformation direction then it becomes possible to reduce or minimize the device size in the deformation direction.